Prevention and protection against electrical shock or leakage currents is a significant consideration in the design of medical electrical equipment. Leakage currents for medical electrical equipment may be defined by the path current takes and can include earth leakage current, enclosure leakage current (or touch current), patient leakage current, patient auxiliary leakage current, and mains voltage to applied part leakage current.
Most medical electrical equipment or devices have contact with a device operator, a patient, or both. Though leakage currents typically are small, the amount of current required to produce adverse physiological effects on a human body is also small, so such leakage currents must be limited to safe values by the design of medical electrical equipment. Accordingly, medical electrical devices must be designed to pass certain tests to ensure that excessive leakage current does not dissipate from the mains, the device enclosure, or applied parts to and through a human body. Portions of Standard ANSI/AAMI/IEC 60601, for instance, address safety requirements for medical electrical equipment.
Medical electrical equipment has a designated class and type, with categorization into class being based on the form of protection used against electrical shock or leakage current and type designation being defined by the degree of protection from electrical shock or leakage current.
Class I medical electrical equipment has a protective earth connection. The primary means of protection for Class I medical electrical equipment is the insulation between “live” parts and exposed conductive parts, such as a metallic enclosure. Supplemental protection is provided by the protective earth connection. Fault or leakage current can flow from the mains to earth via the protective earth conductive connection, which causes a protective device (e.g., a circuit breaker or a fuse) to disconnect the medical electrical equipment from the supply. Note, of course, that not all medical electrical equipment having a protective earth connection necessarily is classified as Class I medical electrical equipment.
Class II medical electrical equipment, on the other hand, does not have a protective earth, and protection against electrical shock is provided by reinforced insulation or double insulation. For double insulation, primary protection is provided by a first layer of insulation (including air) and secondary protection is provided by a second insulation layer. Leakage current can flow from Class II medical electrical equipment.
Different types of medical electrical equipment include B, BF, and CF and each type can afford a different degree of protection against electrical shock or leakage current. Generally speaking, B is for medical electrical equipment providing a particular degree of protection against electrical shock, particularly regarding allowable leakage currents and reliability of the protective earth connection (if present). BF is as type B, but with isolated or floating (F-type) applied part or parts. CF provides a higher degree of protection against electrical shock than BF, particularly with regard to allowable leakage currents and has floating applied parts. For instance, a Class II CF type medical electrical equipment applied part may be required to be designed to allow leakage current of less than 10 μA. Incidentally, an applied part may be defined as a part of the medical electrical equipment which in normal use necessarily comes into physical contact with the patient for the equipment to perform its function or can be brought into contact with the patient or needs to be touched by the patient.
Leakage current can result due to capacitance between the AC supply and the patient. A low dielectric constant, low surface area, and large spacing are common design requirements for minimizing the flow of leakage current.